In recent years, monitoring a biological state of a driver during vehicle driving has drawn attention as an accident prevention measure. The present applicant also discloses a system which is provided with an airbag having a three-dimensional solid knitted fabric inserted therein, where the airbag is disposed at a site corresponding to, for example, a human lumbar area, an air pressure fluctuation of the airbag is measured and human's biological signals are detected from time-series data of the air pressure fluctuation obtained, so that a human's biological state is analyzed. In Non-Patent Literatures 1 and 2, further, trials where an air pack sensor is disposed along a lumbar iliocostal muscle to detect a human's biological signal have been reported.